Who would have thought?

We think of chalk streams as flowing through beautiful open countryside, but there are chalk streams running through urban areas. The River Wandle is one such river and was once fished by Isaak Walton and Lord Nelson. The source is in the chalk of the North Downs and after meandering through the urban sprawl of south London, it joins the Thames in Wandsworth.
It has been restored in recent years and the shoal of chub in the image above was taken at Morden Hall Park which is run by the National Trust.